
INTRODUCTION TO GRAPHS 165
TRY THESE
(ii) Mark number of litres along the horizontal axis.
(iii) Mark cost of petrol along the vertical axis.
(iv) Plot the points: (10,500), (15,750), (20,1000), (25,1250).
(v) Join the points.
We find that the graph is a line. (It is a linear graph). Why does this graph pass through
the origin? Think about it.
This graph can help us to estimate a few things. Suppose we want to find the amount
needed to buy 12 litres of petrol. Locate 12 on the horizontal axis.
Follow the vertical line through 12 till you meet the graph at P (say).
From P you take a horizontal line to meet the vertical axis. This meeting point provides
the answer.
This is the graph of a situation in which two quantities, are in direct variation. (How ?).
In such situations, the graphs will always be linear.
In the above example, use the graph to find how much petrol can be purchased
for ' 800.
Example 4: (Principal and Simple Interest)
A bank gives 10% Simple Interest (S.I.) on deposits by senior citizens. Draw a graph to
illustrate the relation between the sum deposited and simple interest earned. Find from
your graph
(a) the annual interest obtainable for an investment of ' 250.
(b) the investment one has to make to get an annual simple interest of ' 70.
Solution:
Sum deposited Simple interest for a year
' 100 '
= ' 10
' 200 '
= ' 20
' 300 '
= ' 30
' 500 '
= ' 50
' 1000 ' 100
Steps to follow:
1. Find the quantities to be
plotted as Deposit and SI.
2. Decide the quantities to be
taken on x-axis and on
y-axis.
3. Choose a scale.
4. Plot points.
5. Join the points.